1. Field of the Invention
This invention relates to electrical devices comprising PTC conductive polymers.
2. Introduction to the Invention
Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them, are well known. Particularly useful devices comprising PTC conductive. Polymers are self-regulating heaters and circuit protection devices. Self-regulating heaters are hot and have relatively high resistance under normal operating conditions. Circuit protection devices are relatively cold and have a relatively low resistance under normal operating conditions, but are "tripped", i.e., converted into a high resistance state, when a fault condition, e.g., excessive current or temperature, occurs. When the device is tripped by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature at which it is in a high resistance state. Circuit protection devices and PTC conductive polymer compositions for use in them, are described for example in U.S. Pat. Nos. 4,237,411, 4,238,812, 4,255,698, 4,315,237, 4,317,027, 4,329,726, 4,352,083, 4,413,301, 4,450,496, 4,475,138, 4,481,498, and 4,562,313; and in copending, commonly assignedd U.S. application Ser. Nos. 141,989 and 628,945. Other applications which are related to this application are the copending, commonly assigned applications filed contemporaneously with this application by Deep et al, Ser. No. 711,909, by Carlomagno Ser. No. 711,790, by Ratell, Ser. No. 711,907, and by Ratell, Ser. No. 711,908. The disclosure of each of these patents and prior filed pending applications is incorporated herein by reference.
In many devices, and especially in circuit protection devices, it is desirable or necessary for the PTC conductive polymer to be cross-linked, preferably by means of radiation. The effect of the cross-linking depends on, among other things, the polymer and the conditions during the cross-linking step, in particular the extent of the cross-linking, as as discussed for example in copending commonly assigned U.S. application Ser. No. 468,768, the disclosure of which is incorporated herein by reference. When a conductive polymer element is irradiated, the radiation dose absorbed by a particular part of the element in a given time depends upon its distance from the surface of the element exposed to the source, and the intensity, energy and type of the radiation. For a relatively thin element and a highly penetrating source (e.g. a Cobalt 60 source), the variation of dose with thickness is negligible. However, when using an electron beam, the variation in dose with thickness can be substantial; this variation can be offset by exposing the element to radiation from different directions, e.g. by traversing the element past the source twice, irradiating it first on one side and then on the other. Depending upon the energy of the beam and the thickness of the element (which can of course vary, depending upon its shape), the radiation dose can be higher at the surfaces exposed to radiation than at the middle, or substantially uniform across the thickness of the element, or higher at the middle than at the surfaces exposed to radiation. In addition, the radiation dose near the surface exposed to the radiation can be less than expected because of surface scattering, and the radiation dose in the vicinity of the electrodes is affected by the shielding effect and the scattering effect of the electrodes.